Conventionally, as illustrated in FIGS. 10(a) and 10(b), a glaze layer 2 is provided as a heat accumulation layer on an insulative substrate 1 such as that made of ceramic material. Then, film formations are performed by sputtering or deposition from a heat generating resistor material such as that of Ta system, silicide system, Ni-Cr system or the like and an electrode material such as that of Al, Cr-Cu or Au, whereupon a heat generating resistor 3 and wiring electrodes 12 composed of a common electrode and individual electrodes are formed through performance of patterning in the photolithographing step. Thereafter, in order to prevent oxidation of and provide wear resistance with respect to the heat generating resistor 3, a protective film 9 such as that made of SiO.sub.2, Ta.sub.2 O.sub.5, SiAlON, Si.sub.3 N.sub.4 or SiC is formed by sputtering, ion plating or CVD (Chemical Vapor Deposition) to thereby manufacture a thermal head.
However, in the conventional method for manufacturing a thermal head, since the sectional configuration of a peripheral edge portion of each of the wiring electrodes 12 composed of a common electrode and individual electrodes is almost orthogonal, similar difference in level occurs also in the surface of the protective film 9. In addition, due to a difference in growth process between a protective film for the heat generating resistor 3 and that for the wiring electrode 12 when this protective film is formed, a fault 10 at which the film continuity as viewed in the plane direction is discontinued occurs in the protective film layer.
For this reason, the thermal head that had been manufactured by the above-mentioned manufacturing method had its resistance value increased early in its use, with the result that when printing was performed using this thermal head, such increase in resistance value became a cause of dotting failures, which resulted in that the printing run service life of the thermal head became shorter. Also, it is considered that during printing run, ions in the thermosensible paper, moisture, Na.sup.+ ion and Cl.sup.- ion in the atmosphere, and the like enter into the thermal head due to the faults 10 of the protective film thereof, with the result that there was the problem that the heat generating resistor 3 and wiring electrodes 12 were corroded and as a result the thermal head had inferior corrosion resistance.
As conventional examples of solving the above-mentioned problems, a manufacturing method wherein a forward end portion of each wiring electrode 12 connected to a heat generating resistor 3 is tapered to thereby decrease the fault and level difference of the protective film (e.g., Published Unexamined Japanese Patent Application No. S-56-129184), a manufacturing method wherein a photo-step and etching step are performed twice or so with respect to a forward end portion of each wiring electrode 12 connected to a heat generating resistor 3 to thereby form this forward end portion into a two-stepped configuration and thereby decrease the level difference of the protective film (e.g., Published Examined Japanese Patent Application No. S-55-30468), a manufacturing method wherein high frequency bias sputtering is added during formation of the protective film to thereby prevent generation of cracks (e.g., Published Unexamined Japanese Patent Application No. S-63-135261), etc. have been made publicly known.
However, while in the conventional thermal head a wiring electrode thereof was such that a specific configuration was imparted to only a forward end portion thereof that is connected to the heat generating resistor, the effect thereof upon enhancement of the printing durability and reliability was not sufficient. Namely, faults and level differences in the protective film that result from the level differences in the wiring electrode occur not only in the portion thereof that corresponds to the forward end portion of the wiring electrode connected to the heat generating resistor but also in the portion thereof that corresponds to an entire peripheral edge portion of the wiring electrode in at least a protective-film region.
On the other hand, if the above-mentioned level differences exist, the protective film 9 is likely to be partly broken off or exfoliated from faults 10 thereof due to mechanical stress that is applied to the level difference portion thereof by sliding movement of the thermosensible paper and pressing force of a platen roller or due to thermal stress that results from a difference in thermal coefficient of expansion between the heat generating resistor portion and the electrode portion. Accordingly, the effect of the sliding movement of the thermosensible paper and pressing force of the platen roller is exerted not only upon the heat generating resistor but also upon the surrounding areas thereof, with the result that the protective film is likely to be broken off or exfoliated also by way of the peripheral edge portion of the wiring electrode other than the forward end portion thereof. Also, even when scratches have been made by foreign substances having attached to the thermosensible paper or the like, these foreign substances are caught by the level difference portion of the wiring electrode, with the result that, similarly, exfoliation or the like of the protective film is likely to occur also from a portion of the electrode other than the forward end thereof.
As mentioned above, the protective film was broken off or exfoliated not only from the forward end portion of the electrode but also from the peripheral edge thereof, whereby the printing run service life of the thermal head was caused to become shorter.
Also, while material having high hardness has on one hand been recently used as material of the protective film in order to improve the wear resistance thereof, emphasis has on the other hand been placed on the above-mentioned problems. Particularly, when cladding is applied using a hard protective film, the thermal head cannot receive an external force with high flexibility and it is also difficult to ease the stress. Accordingly, there was the problem that the phenomenon such as exfoliation or the like of the protective film was likely to become prominent.
Conversely, when the hardness of the protective film is low, the wear resistance becomes inferior, with the result that the heat generating resistor is damaged due to wear of the protective film and therefore the printing run service life can no longer be expected to be improved.
Also, there is the likelihood that during printing run, ions in the thermosensible paper, moisture, Na.sup.+ ion and Cl.sup.- ion in the atmosphere, and the like may enter into the thermal head due to the level difference of the peripheral edge of the electrode. As a result, there was the problem that this entry corroded the heat generating resistor and electrode, with the result that the thermal head became inferior in terms of the corrosion resistance particularly during standby for printing.
Accordingly, an object of the present invention is to provide a thermal head which is arranged such that, in order to solve the above-mentioned conventional problems, the peripheral edge portion of the electrode thereof is tapered and the level difference on the surface of the protective film is thereby lessened to thereby have no fault therein while having wear resistance, on the other hand.